The present invention relates to a method and apparatus for rapid temperature measurement and in particular to a method of operating a temperature measurement probe to rapidly acquire the temperature of a workpiece on co-ordinate positioning apparatus.
It is known that when a temperature sensor is brought into contact with an object, such as a workpiece, it will take a certain amount of time for the temperature sensor to reach the temperature of the object. For applications where the speed of temperature measurement is not critical, it is known to simply keep the temperature sensor in contact with the object for enough time for thermal equilibrium to be reached.
For certain applications it is, however, desirable to measure the temperature of an object rapidly. This is especially the case when measuring the temperature of workpieces or other objects being produced or inspected in an automated production facility. An extrapolation technique for predicting the temperature of a workpiece on a co-ordinate measuring machine has been described previously in U.S. Pat. No. 5,195,826. The method involves providing a temperature sensing probe that includes a switch for sensing when the temperature sensor makes initial contact with the workpiece. Temperature data are then collected for a certain period of time after the initial workpiece contact. A numerical technique is used to extrapolate the temperature data curve to provide an estimate of the equilibrium temperature value.
The technique of U.S. Pat. No. 5,195,826 has the disadvantage of being relatively complex. In particular, the method can be difficult to implement practically because it relies on knowledge of the precise shape of temperature response curve and also the time of initial workpiece contact. The amount of data processing required to provide the numerical prediction can also be prohibitive.
In the medical field, electronic clinical thermometers are known in which an equilibrium body temperature is established using a predictive technique. EP482562, for example, describes a technique for determining body temperature in which collected temperature data are fitted to an equation that allows an equilibrium temperature to be predicted. The equilibrium temperature prediction is continually updated as more temperature data are collected until the temperature prediction is found to be sufficiently accurate. In EP482562, the temperature prediction is held to be sufficiently accurate when the rate of change of the measured temperature values with time drops below a certain predetermined value and when the change in predicted equilibrium temperature with time drops below a certain predetermined value.